Response in ended before the catch in RequestListener

[asadhazara]

In the following example the server is not returning 500 Internal Server Error. Instead the server throws an error (Error [ERR_HTTP_HEADERS_SENT]: Cannot render headers after they are sent to the client) and the process is killed.

import http from 'http';
import { createHandler } from 'graphql-sse';

const handler = createHandler({
  ...
  onNext: () => {
    throw new Error('Not implemented');
  },
});

http.createServer(async (req, res) => {
  try {
    await handler(req, res);
  } catch (err) {
    res.writeHead(500, 'Internal Server Error').end();
  }
});

[enisdenjo]

Hmm, I couldn't manage to replicate your issue, I've added tests describing your case, and they pass.

Can you elaborate further? Which version are you using? How are you connecting to the server? Which client are you using?

Ideally a full repro would be perfect and much appreciated!

[enisdenjo]

Ah, ok, I misread - the error is thrown properly, but you have problems writing the headers.

Writing a 200: OK is necessary in order to establish an SSE connection; meaning, headers are already written and flushed at the point where onNext is reached because there's an active and accepted SSE connection.

[asadhazara]

If I want to write a different head, what are the options? For instance, I would like to send a 500 when something goes wrong in a resolver.

[enisdenjo]

You cannot write a different header. As mentioned before, it is already written and flushed at that point.

You can only leave a log on your server and fix the onNext handler.

[enisdenjo]

Since re-writing the header is not possible, you can catch the error in onNext and populate the errors GraphQL result field to transmit to the client through the next message.

Something around the edges:

import http from 'http';
import { createHandler } from 'graphql-sse';

const handler = createHandler({
  // ... your other options ...
  onNext: () => {
    try {
      mightThrowAnError();
    } catch (err) {
      return {
        errors: [new GraphQLError(err)],
      };
    }
  },
});

http.createServer(async (req, res) => {
  try {
    await handler(req, res);
  } catch (err) {
    if (!res.headersSent) {
      res.writeHead(500, 'Internal Server Error').end();
    } else {
      console.error('Internal Server Error', err);
    }
  }
});

But beware, I do NOT recommend this. You should absolutely make sure that nothing is thrown from onNext. Execution errors, errors that are a part of the next message, are not terminal - the client will not end and the operation will proceed blindly.

[asadhazara]

Thank you @enisdenjo! The example I have used is directly copied from the JSDoc for the Handler. I have submitted a PR for this.

[enisdenjo]

🎉 This issue has been resolved in version 1.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀